Digital cameras typically store captured images digitally, and generally perform storage using a compressed form of the image, where a degree of compression is selected in order to trade-off storage capacity against reconstructed image quality. Traditionally, images are compressed on the camera using the Joint Photographic Expert Group (JPEG) still image compression standard. A quality setting is typically selected prior to capturing the image. Higher quality settings result in relatively higher reconstructed image quality, at the cost of higher consumption of storage space. Conversely lower quality settings require less storage space for the image, but result in lower reconstructed image quality. Therefore, low quality images require less storage space, allowing more low quality images to be stored on a memory device in the camera. Accordingly, there is a trade-off between image quality and a maximum number of images which can be simultaneously stored on the memory device in the camera.
FIG. 1 shows a prior art digital camera process flow, indicating how quality selection is an encode-time choice, which is made before the image is taken. The figure depicts a discrete image capture process 108 whereby a quality parameter is selected in a step 100. Thereafter, an image capture step 102, is followed by an image compression and storage step 104. The image capture process 108 is thereafter directed in accordance with an arrow 106 back to the quality parameter selection step 100. An initial choice of low quality cannot be reversed after the image has been taken, and accordingly a digital camera user may be inclined to use high quality more often than not. This however results in rapid exhaustion of on-board image storage capacity.
Some traditional applications using digital images, for example digital printing, have reduced the size of digital image files, which have been compressed at a particular quality, by first decompressing, and then re-compressing the image again at a lower quality. This is, however, a slow and computation intensive approach, not suitable for use in portable electronic equipment such as digital cameras. Furthermore, decompression and subsequent recompression introduces other problems such as loss of quality.